The present invention relates to a data-reproducing. System for digital recording and reproducing devices using the PBML signal-processing technique, such as hard disk drives. More specifically, the present invention relates to a preamble pattern for initial acquisition in AGC and PLL processing in front end processing and to an initial acquisition procedure using the preamble pattern.
The conventional magnetic-recording channel is of the so-called longitudinal recording system type. According to this system, information is recorded in the form of plus magnetization and minus magnetization arranged parallel to the recording surface. Various pieces of information are recorded on magnetic disk media of the longitudinal recording system type in accordance with a format such as is shown in FIG. 14. The servo pattern is provided in a region to record a bit pattern for tracking and addressing, which is used to position the magnetic head. The magnetic head records and reproduces data based on the bit pattern. A gap is inserted after the servo pattern to cope with a variation of the rotation of the disk, and a dataxcx9csector follows the gap. The data sector is a region where a certain pattern called a xe2x80x9cpreamblexe2x80x9d is recorded repeatedly. By referring to this region, initial acquisition by an AGC circuit and a PLL circuit is carried out. Thereafter, information of a synchronization pattern (SYNC) is read, and the data is reproduced.
In the recording format of a hard disk drive, the preamble region is indispensable for reproducing data, but the provision of this preamble region is one of the factors reducing the data efficiency; accordingly, it is preferable to keep the preamble region as small as possible. A number of methods have been proposed from various viewpoints to control the amplitude and timing accurately, using as small a number of bits as possible.
In U.S. Pat. No. 5,258,933, there is disclosed a technique for making use of a zero-cross comparator to convert a signal reproduced from a preamble into binary codes. A zero-cross timing pulse is obtained by the zero-cross comparator, and this timing pulse is fed to a VCO to stop its oscillation for a certain time. Then the VCO resumes oscillation one half of the duration of one bit after the next zero-cross timing pulse. Thus, the phase shift between the reproduced signal and the VCO clock is reduced to almost zero to shorten the time necessary for phase acquisition.
U.S. Pat. No. 5,552,942 discloses another technique. According to this technique, the phase of a VCO clock is optimally controlled by calculating the mean square errors of expected values and actual values of samples of a known preamble pattern.
Japanese Patent Laid-Open No. 315517/1996 discloses a technique for expanding the capture range to xc2x1T (T is the duration of one bit) as effective measures in case the SN (signal-to-noise) ratio of the preamble pattern goes down.
According to the conventional timing-controlling method, a timing gradient is calculated from the amplitude value X(k) at the k-th sampling point and the judged value r(k) of X(k) by using the following expression:
xcex94xcfx84(k)=xe2x88x924(kxe2x88x921)xc2x7X(k)+r(k)xc2x7X(kxe2x88x921)
and the timing of the VCO clock is renewed so as to converge the result to zero. The ideal capture range is xc2x1T/2. However, if r(k) is judged erroneously due to noise, the timing gradient may exceed the capture range of xc2x1T/2, elongating the time necessary for phase acquisition. To prevent it, r(k) is not judged at every sampling point, but is predicted by making use of the periodicity of the preamble pattern. If the preamble pattern is of a 4T cycle, r(k) has the repetition of (+1, +1, xe2x88x921, xe2x88x921); therefore, the timing gradient at every second sampling point is given by the expression below.
xcex94xcfx84(k)=X(k)xe2x88x92X(kxe2x88x922)
Japanese Patent Laid-Open No. 21096/2000 discloses a technique to obtain a timing clock for sampling by-using a preamble pattern which is not of a 4T cycle. This technique relates to the PR5 equalization system, which is suitable to magnetic-recording channels for reproducing data of a single-layer perpendicular-recording medium with a ring head. A timing clock for sampling is obtained by using a preamble pattern consisting of repetition of a magnetization pattern of xe2x80x9c+1, +1, +1, xe2x88x921, xe2x88x921, xe2x88x921xe2x80x9d in PBML processing, wherein a single-layer perpendicular-recording medium and a ring head are combined with the PR5 system.
The development of a perpendicular-recording system has been accelerating recently, and there is a good possibility that most media will shift from the longitudinal recording system to the perpendicular-recording system because the latter is suitable to high-density recording.
Unlike the longitudinal recording channel which shows band-pass filter characteristics, the response of reproduced signals of a perpendicular-recording medium has a spectrum in a low-frequency area adjacent to DC, and it is known that the waveform responding to isolated transition of perpendicular magnetization can be well approximated by the hyperbolic tangent function tanh (x).
Due to improvement of the recording density and of the reproduction sensitivity by GMR heads and so on in recent magnetic systems, there is a tendency for the noise arising from recording media to become more salient than the noise arising from the circuits of systems. An example of the medium noise is jitter-like noise due to the variation of the transition point in magnetization. As the jitter-like noise becomes predominant over the system noise, which is white noise, it becomes difficult to improve the performance of hard disk drives of the PRML that make making use of a spectrum of lower frequency.
Desirable in a perpendicular magnetic recording system, wherein a perpendicular-recording two-layer medium and a single magnetic-pole head are combined, is the PR(1, 1) system, the PR(1, 2, 1) system, or the PRML system, which is based on the former two systems, and wherein equalization is effected according to the nature of the noise on the channel and data is demodulated by an ML, each system being capable of using the spectrum in the low-frequency area effectively. In this case, the provision of means for controlling timingxcx9c and amplitude to sample a reproduced signal at proper timing and amplitude is indispensable.
The most simple method of accomplishing the above is as follows. A reproduced signal with a spectrum of lower frequency and a differentiated signal obtained by differentiating the reproduced signal are prepared. The timing for sampling is controlled and the gain is adjusted by applying a circuit of initial acquisition by a conventional 4T-cycle preamble pattern to the differentiated signal, and the reproduced signal with a spectrum of lower frequency is sampled with a phase-controlled clock. In the case of this method, it is necessary to provide means for switching between the reproduced signal with a spectrum of lower frequency and the differentiated signal, as well as a means for correcting the relative time difference between the reproduced signal with a spectrum of lower frequency and the differentiated signal.
If there is heavy jitter-like noise due to variation of the point of magnetization transition, the amplitude levels at sampling points fluctuate to cause large errors in computed timing and amplitude gradients. There are four sampling points in one cycle of a 4T-cycle preamble. If white noise is added to it, averaging can be effected four times a cycle, and hence the variance of timing gradients-is reduced to 1/4=xc2xd. On the other hand, because jitter-like noise is caused by the displacement of the point of magnetization transition, both the former two sampling points and the latter two sampling points are saliently correlated and tend to move at a level in the same direction; accordingly, averaging can be effected only two times a cycle in substance. Thus, if jitter-like noise is predominant over white noise, the errors of computed timing and amplitude gradients are difficult to reduce. If the loop gain is set high at an early stage of acquisition to reduce the time necessary for the acquisition in such a channel, the set values of the VCO and the VGA may be affected considerably when the loop gain is switched to another value. In this case, if the loop gain is switched to the former value, the acquisition does not. finish in the remaining part of the preamble; therefore, the loop gain can not be set high in acquisition, which makes the acquiring time long, increases the number of bits to be allotted to the preamble pattern, and reduces the data efficiency.
In the case of the technique of zero-phase start disclosed in the U.S. Pat. No. 5,552,942, because the mean square error has to be calculated a number of times by changing the timing of sampling to find a timing which gives a minimum mean square error, a large number of bits have to be allotted to the preamble pattern.
In the case of the technique of extracting a clock for the correct timing of sampling by using a 6T-cycle preamble pattern disclosed in the Japanese Patent Laid-Open No. 2000-21096, the effects of jitter-like noise are not dealt with. Because signals after equalization by the PR5 system are used and a magnetic-recording channel with band-pass filter characteristics similar to the PR4""s characteristics is assumed, it is difficult to reduce the number of bits to be allotted to the preamble.
The above two techniques deal with the extraction of the timing clock, but do not deal with the effects ofxcx9c!jitter-like noise in amplitude adjustment.
A first object of the present invention is to provide (i) a preamble pattern to reduce the effects of jitter-like noise in timing and amplitude control which is indispensable in a signal-processing technique for perpendicular-recording hard disk drives using magnetic-recording media with a spectrum of lower frequency, and (ii) an acquisition system using the preamble pattern.
A second object of the present invention is to provide a signal-processing system which (i) prevents a transitional response from occurring in the reproduced signal when the reproduced signal is switched from a DC-erased state to the preamble pattern while the DC component is being removed from a reproduced signal with a spectrum of lower frequency by using a high-pass filter, and (ii) makes effective use of the spectrum of lower frequency.
A third object of the present invention is to provide (i) a preamble pattern to reduce the effects of jitter-like noise in timing and amplitude control of hard disk drives using longitudinal magnetic-recording media, and (ii) an acquisition system using the preamble pattern.
Jitter-like noise is caused by random displacement of the point of magnetization transition. As shown in FIG. 16, a waveform responding to isolated magnetization transition of a perpendicular-recording magnetic channel with a lower-frequency spectrum can be approximated by the hyperbolic tangent function, and a reproduced signal of any pattern can be obtained by superposing response waveforms on one another. When the point of isolated magnetization transition is displaced forward or backward, the nearer the sampling point is to the transitional point, the larger the amplitude change is. This is apparent from the differentiated waveform of the response waveform. To avoid the effects of jitter-like noise, it is effective to use amplitude data obtained at points speed from the transitional point. In the case of a conventional 4T-cycle preamble,-sampling points (xe2x97xaf) 0.5T away from the transitional point are used; accordingly, the effects of jitter-like noise are large in the timing and amplitude control. If sampling points (∘) 1.5T away from the transitional point can be used, the differential values are reduced to a fifth, reducing the effects of jitter-like noise significantly. On the other hand, as shown in FIG. 18, the waveform responding to isolated magnetization transition of the conventional longitudinal magnetic-recording channel can be approximated by the Lorentz waveform, and a reproduced signal of any pattern can be obtained by superposing, response waveforms on one another. The amplitude change at the time of the isolated transitional point being displaced forward or backward is spaced a maximum of 0.5T to 1.0T away from the transitional point. This is apparent from the differentiated waveform of the response waveform. To reduce the effects of jitter-like noise, it is effective to avoid the peak area of the differentiated waveform and use the amplitude data obtained at the transitional point. In the case of a conventional 4T-cycle preamble, amplitude data at the points (∘) 0.5T away from the transitional point are used; accordingly, the effects of jitter-like noise are large in the timing and amplitude control. If amplitude data at the sampling point (xe2x97xaf) whose phase is the same as the phase of. the transitional point can be used, the effects of jitter-like noise are reduced significantly.
The first aspect of this invention for solving the above problems relates to a magnetic-recording channel with a spectrum of lower frequency. According to this aspect, amplitude data to be obtained at sampling points spaced 1.5T or more away from the point of magnetization transition of a xe2x88x926T or longer-cycle preamble pattern are used. At the sample points, the differential values of amplitude are small, and hence, the amplitude changes due to a certain phase shift of sampling are relatively small; therefore, stable amplitude control can be accomplished if amplitude gradients for amplitude control are computed at the sampling points. In addition, because the amplitude reaches its peak at the sampling points, the amplitude gradients are maximum at the sampling points though samples per one cycle are few in number; therefore, acquisition can be completed in a short time.
The second aspect of this invention for solving ,the above problems relates to a magnetic-recording channel with a spectrum of band-pass filter characteristics. According to this aspect, sampling is carried out at the point of magnetization transition and the amplitude data obtained by the sampling is used. At xc3xa6 the sample point, the differential value of amplitude is small, and hence, the amplitude change due to a certain phase shift of sampling is relatively small; therefore, stable amplitude control can be accomplished if the amplitude gradient for amplitude control is computed at the sampling point. In addition, because the amplitude reaches its peak at the sampling point, the-amplitude gradient is maximum at the sampling point though samples per one cycle are few in number; therefore, acquisition can be completed in a short time.
According to the third aspect of this invention for solving the above problems, sampling is carried by a VCO clock signal sent out at a predetermined phase, the phase error is computed by using the amplitude data obtained by the sampling, the phase shift between the VCO clock signal and the reproduced signal of the preamble pattern is estimated, and the phase of the VCO is adjusted optimally, instead of setting the loop gain high and completing the acquisition in a short time.
To be more specific, sampling points are set in at least, one cycle of the preamble pattern by initial phase setting of the VCO. The timing gradient at each sampling point is computed to search for a sampling point for initiating a zero-phase start. Applied to the computation of timing gradients is the prior art disclosed in the Japanese Patent Laid-Open No. 315517/1996 of enlarging the phase-capture range by making use of the periodicity of a preamble pattern. After the search of the point for initiating a zero-phase start, the absolute value of the phase difference at the initiating point is estimated by using the timing gradients at the other sampling points too. And, the phase of the VCO was adjusted at a time based on the estimated value. The prior art disclosed in the Japanese Patent Laid-Open No. 249752/1996 was improved to estimate the phase difference. By applying the improved art to timing gradients having the same periodicity as the preamble pattern, the phase difference is estimated from the magnitudes of computed timing gradients at the sampling points. In the case of the zero-phase start system of the present invention, the initial phase adjustment is completed by sampling one cycle of the preamble pattern and adjusting the phase of the VCO once; therefore, the system does not require many preamble patterns. After the initial phase adjustment, a feedback loop using usual timing gradients is started at a low loop gain. Thus, although a phase error due to jitter-like noise remains in the initially adjusted phase of the VCO, acquisition can be started with a minimum phase error. Because the loop gain in acquisition can be set low enough, taking jitter-like noise into account, the problem of the jitter-like noise causing the phase of the VCO to vary can be solved.
To attain the above second Object, a DC-free pattern, such as a 4Tor shorter-cycle pattern, is recorded in a region between a servo pattern and a data sector.